High Performance Web Sites

The Performance Golden Rule

This phenomenon of spending only 10–20% of the response time downloading the HTML document is not isolated to Yahoo!'s home page. This statistic holds true for all of the Yahoo! properties I've analyzed (except for Yahoo! Search because of the small number of components in the page). Furthermore, this statistic is true across most web sites. Table 1-1 shows 10 top U.S. web sites extracted from http://www.alexa.com. Note that all of these except AOL were in the top 10 U.S. web sites. Craigslist.org was in the top 10, but its pages have little to no images, scripts, and stylesheets, and thus was a poor example to use. So, I chose to include AOL in its place.

Table 1-1. Percentage of time spent downloading the HTML document for 10 top web sites

Â	Empty cache	Primed cache
AOL	6%	14%
Amazon	18%	14%
CNN	19%	8%
eBay	2%	8%
Google	14%	36%
MSN	3%	5%
MySpace	4%	14%
Wikipedia	20%	12%
Yahoo!	5%	12%
YouTube	3%	5%

All of these web sites spend less than 20% of the total response time retrieving the HTML document. The one exception is Google in the primed cache scenario. This is because http://www.google.com had only six components, and all but one were configured to be cached by the browser. On subsequent page views, with all those components cached, the only HTTP requests were for the HTML document and an image beacon.

In any optimization effort, it's critical to profile current performance to identify where you can achieve the greatest improvements. It's clear that the place to focus is frontend performance.

First, there is more potential for improvement in focusing on the frontend. If we were able to cut backend response times in half, the end user response time would decrease only 5–10% overall. If, instead, we reduce the frontend performance by half, we would reduce overall response times by 40–45%.

Second, frontend improvements typically require less time and fewer resources. Reducing backend latency involves projects such as redesigning application architecture and code, finding and optimizing critical code paths, adding or modifying hardware, distributing databases, etc. These projects take weeks or months. Most of the frontend performance improvements described in the following chapters involve best practices, such as changing web server configuration files (Chapter 5 and Chapter 6); placing scripts and stylesheets in certain places within the page (Chapter 7 and Chapter 8); and combining images, scripts, and stylesheets (Chapter 3). These projects take hours or days—much less than the time required for most backend improvements.

Third, frontend performance tuning has been proven to work. Over 50 teams at Yahoo! have reduced their end user response times by following the best practices described here, many by 25% or more. In some cases, we've had to go beyond these rules and identify improvements more specific to the site being analyzed, but generally, it's possible to achieve a 25% or greater reduction just by following these best practices.

At the beginning of every new performance improvement project, I draw a picture like that shown in Figure 1-1 and explain the Performance Golden Rule:

Only 10–20% of the end user response time is spent downloading the HTML document. The other 80–90% is spent downloading all the components in the page.

The rest of this book offers precise guidelines for reducing that 80–90% of end user response time. In demonstrating this, I'll cover a wide span of technologies: HTTP headers, JavaScript, CSS, Apache, and more.

Because some of the basic aspects of HTTP are necessary to understand parts of the book, I highlight them in Chapter 2.

After that come the 14 rules for faster performance, each in its own chapter. The rules are listed in general order of priority. A rule's applicability to your specific web site may vary. For example, Rule 2 is more appropriate for commercial web sites and less feasible for personal web pages. If you follow all the rules that are applicable to your web site, you'll make your pages 25–50% faster and improve the user experience. The last part of the book shows how to analyze the 10 top U.S. web sites from a performance perspective.